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Beta-type titanium alloys have a low elastic modulus, excellent cold workability, and are widely used as implant materials. High strength is possible by forming a precipitation in the β-matrix via solution treatment and aging treatment. However, beta titanium alloys require a large amount of beta-stabilizing elements (Fe, Nb, Mo, Ta, etc), and these expensive beta stabilizing elements increase the manufacturing cost of these alloys. In this study, Ti-5Mo-2Fe metastable beta-titanium alloy was designed by adding Mo and Fe, which were relatively inexpensive and had excellent biocompatibility among beta-stabilizing elements, and an ingot was manufactured by vacuum arc remelting. Solution treatment was maintained at 850℃ for a holding time of 1 hour, followed by furnace cooling. Aging treatments were conducted in a range of temperature 350~500℃ and holding time 2 h~48 h. The microstructure behaviors and mechanical properties were analyzed according to these aging treatment conditions. Isothermal ω phases were precipitated by aging treatment, and hardness and yield strength were found to be significantly higher for conditions of 400℃, 8h. As holding time and temperature increased, these phases transformed into secondary alpha phases and the hardness and yield strength decreased due to this microstructural evolution. (Received 8 February, 2023; Accepted 17 May, 2023)

keyword : Metastable beta titanium, aging treatment, ω phase, secondary α phase

The localized corrosion behavior of precipitation-hardened UNS N07718 was investigated by immersion tests in 6 wt% FeCl₃ + 1.0 wt% HCl and the use of electrochemical techniques in a simulating solution of a diluted-sour environment of 25 wt% NaCl + 0.5 wt% CH₃COOH. The Ti carbides and Nb-Mo carbides with 1-10 μm size were distributed in the alloy. After immersion at a solution temperature higher than 45℃, localized corrosion with a depth of over 25 μm was identified, and the critical pitting temperature was determined to be 45℃. Potentiodynamic polarization showed that the surface of the UNS N07718 was immediately passivated in the experimental solution. The passivity-maintaining current density was gradually increased with increasing solution temperature, and finally, localized corrosion was initiated or propagated at 0.5 V_SSE in 80℃. The localized corrosion was initiated or propagated at the interface between the Ti and Nb-Mo carbides and the alloy substrate. Scanning Kelvin probe microscopic images revealed that the contact-potential difference values were in the order of Ti carbide > Nb-Mo carbide > alloy substrate, indicating that the carbides and alloy substrate act as a cathode and an anode, respectively, forming a micro-galvanic couple. Therefore, it is concluded that localized corrosion is initiated at the interface between the carbides and substrates in UNS N07718. (Received 15 September, 2022; Accepted 8 May, 2023)

keyword : localized corrosion, UNS N07718, carbide, scanning kelvin probe, acidic chloride, diluted-sour environment

There has been an increased demand for surface modification technologies to enhance corrosionresistance and wear-resistance. Among the representative surface coating technologies, laser cladding has attracted great attention because of its multiple advantages including low heat input, low dilution ratio, controllable clad height, etc. Recently, laser cladding has been considered as a surface coating technology for the next-generation small modular reactor, although submerged arc welding was utilized for the 3rd generation nuclear reactor. Cobalt-free materials are required as cladding materials in the nuclear reactor because cobalt has a long half-life. In this work, nickel based cobalt-free Alloy 82 powder was utilized. The main experimental parameters for laser cladding were intensively investigated by varying laser power, scan speed, powder supply, carrier and shield gas, overlap ratio, etc. Additionally, cross-sectional area calculation and EPMA analysis were carried out to examine the dilution ratio. Mechanical properties were also evaluated using microhardness tests and wear tests at high temperatures. Finally, corrosion tests were performed to compare a laser clad surface with an uncoated carbon steel surface. Our study indicates that the laser cladding using Alloy 82 powder is feasible for corrosion protection in next-generation small modular reactors. (Received 17 March, 2023; Accepted 4 May, 2023)

keyword : laser cladding, dissimilar cladding, Alloy 82, small modular reactor, corrosion-resistance

On the road to carbon neutrality, a great deal of attention is being paid to emerging technologies such as the DRI (direct reduced iron) process. This study proposes a two-stage DRI process using reformed COG (coke oven gas), and determined optimal process parameters. The reduction and carbonization of Carajás iron ore used in the field were examined by monitoring the weight loss of the samples, and EDS and XRD measurements with respect to the reaction temperature and operating time for different reducing environments. While the reduction of iron ore is completed in 60 min at 800 ℃ regardless of the reducing environment, the carbonization of reduced iron is attainable only at 800 ℃ with high hydrogen content in the reducing gas. Thus, a countercurrent scheme in the proposed DRI process is justified, in which COG containing high hydrogen content is flowed into the 2nd stage operated at 800 ℃ and subsequently directed to the 1^st stage operated at 600 ℃. The reduction of iron ore is initiated in the 1^st stage for 60 min irrespective of the reducing environment, and the completion of the reduction and the following carbonization is fulfilled in the 2^nd stage for 40 min under a high reducing environment. An equilibrium analysis supported that the cracking of CH₄ in COG to graphite leads to the formation of CO from CO₂ and the successive formation of Fe₃C from reduced Fe. The carbonization of iron ore is possible only in the presence of CO₂. It also showed that too high or low reducing environments are not desirable to accomplish the DRI process. This study is expected to be provide an effective guideline for optimizing similar DRI processes. (Received 16 November, 2022; Accepted 17 May, 2023)

keyword : DRI, direct reduced iron, COG, coke oven gas, GOD, gas oxidation degree, iron carbide, Fe3C, cementite

The potential applications of advanced electronic materials in large-area, printable, and flexible electronics have generated significant interest. However, creating high-performance, low-voltage thin-film transistors (TFTs) for these applications remains difficult due to a lack of advanced gate dielectric and semiconductor materials that meet both ease-of-fabrication requirements and high electrical performance. In this study, we present high-performance gate dielectric thin-films, which were fabricated using a facile solution-based technique, and then employed to realize low operating voltage organic and metal oxide semiconductor-based thin-film transistors. The high-k oxide gate dielectrics were produced via a simple dipcoating method, resulting in the formation of thin-oxide layers. These novel oxide gate dielectrics demonstrated exceptional dielectric properties, with large capacitances (up to 430 nF/ cm²), low-level leakage current densities (< 3 × 10^-8A/cm² at 4 V), featureless morphology (rms roughness < 0.36 nm), and high transparency ( > 85%). Consequently, these dip-coated gate dielectrics can be incorporated into thin-film transistors, utilizing pentacene as p-type organic semiconductors. Furthermore, by employing dip-coating, indium oxide and indium-gallium-zinc oxide can be utilized as n-type inorganic semiconductors, allowing for the fabrication of low-voltage operation and high-performance inorganic TFTs. The resulting TFTs functioned at ultralow voltages (< ± 2 V) and achieved high transistor performance (hole mobility: 0.28 cm²V^-1·s^-1, electron mobility: ~2.0 cm²V^-1·s^-1 and on/off current ratio >10⁵). (Received 13 March, 2023; Accepted 2 May, 2023)

keyword : dip-coating, gate dielectric, semiconductor, solution-process, thin-film transistor, low-voltage operation

Oxide thin-film transistors (TFTs) are important semiconductor materials for display backplanes. To fabricate flexible displays, not only display modes but also TFTs are important. Flexible TFTs are especially needed for flexible displays, and related processes need to be developed. Printing is a good choice for the new fabrication of oxide TFTs. Electrohydrodynamic (EHD) jet printing is an excellent alternative for making flexible TFTs. To improve the electrical properties of oxide TFTs using EHD jet printing, propylene monomethyl ether acetate (PGMEA) was added to a zinc-tin oxide (ZTO) formulation. EHD jet printing was performed by Taylor cone jet mode with parameters of 2.4 kV and 0.064 μL/s to obtain uniform thin films at a substrate temperature of 50℃. Much improved TFT properties were obtained, including a mobility of 7.11 cm²/V s, on-to-off current ratio of 2.8 × 10⁶ and subthreshold slope of 1.44 V/dec^-1 for ZTO TFT with 5 wt% of PGMEA, and a mobility of 1.43 cm²/V s, on-to-off current ratio of 2.7 × 10⁵ and subthreshold slope of 1.32 V/dec^-1 for the ZTO TFT. Almost no hysteresis behavior was observed in the oxide TFTs with added PGMEA. We report a new way to improve the electrical properties of oxide TFTs, by the simple addition of PGMEA. (Received 20 March, 2023; Accepted 4 May, 2023)

keyword : EHD jet, oxide TFT, additive

Thermoelectric energy harvesting has attracted a lot of attention for powering self-powered devices because of the potential to generate energy anywhere with a temperature difference. In particular, a stretchable thermoelectric generator (S-TEG) can be applied to the repetitively moving parts of a machine and even a human body. Herein, we suggested a S-TEG using thermoelectric composite films made by dispersing n-type Bi₂Te_2.7Se_0.3 powders into the polyvinylidene fluoride elastomer. The prepared n-type thermoelectric composite film with 75 wt% of Bi₂Te_2.7Se_0.3 powders showed a power factor of 1.81 mW m^-1 K^-2 at room temperature. Next, we fabricated S-TEG by encapsulating thermoelectric powders-based composite films, and Ag-coated textile electrodes with an Eco-flex matrix. The fabricated stretchable energy harvester generated a maximum output power of 2.35 nW at a temperature difference (ΔT) of 25 K. By repeatedly introducing ΔT=5K, our S-TEG converted the output voltage of 3.4 mV and current signals of 0.25 mA. Moreover, a finite element analysis with multiphysics COMSOL simulation software was conducted to compare the experimental and theoretical thermoelectric output performance of the fabricated S-TEG. Finally, we demonstrated energy harvesting by converting human body heat into electrical energy for potential utilization of our energy harvester. This study led to the development of a S-TEG design using thermoelectric film with a simple and low-cost fabrication procedure, providing a potential approach for use as a next-generation wearable device power source. (Received 21 April, 2023; Accepted 12 June, 2023)

keyword : thermoelectric, energy harvesting, flexible, stretchable, thermoelectric generator

Recently, transition metal dichalcogenides (TMDCs) with 2D structure have attracted interest due to their many unique optical and electrical properties. The primary preparation methods for 2D materials are chemical vapor deposition (CVD), exfoliation, and other vacuum technologies. Large-scale synthesis of WS₂ via solution-process is rare due to the higher temperature needed for tungsten-based precursors. Combination of spin coating or dip coating with CVD have been studied recently to make large-area 2D TMDC with good electrical properties. Here, we report a new synthetic route for large WS₂ crystal that combined solution coatings and CVD process. A solution of sodium tungstate and hydrazine hydrate with sodium thiosulphate was coated on a silicon wafer via dip and spin coating. The films were then treated with CVD at various positions and temperatures to facilitate crystallization. The double coating conditions and CVD parameters were modified to obtain WS₂ crystals. Triangular shaped 44 ± 4 μm WS₂ crystals could be obtained with simple annealing above 900℃ without gas treatment. The synthesized WS₂ was found to be bulk with a triangular shape, as confirmed by Raman and AFM analyses. A PL peak of WS₂ at 643 nm was observed at an early crystallization stage. (Received 4 May, 2023; Accepted 27 June, 2023)

keyword : 2D, TMDC, WS_{2, CVD, solution process}

YbCd₂Sb₂-based Zintl phases have been identified as promising materials for thermoelectric applications due to their high Seebeck coefficient and electrical conductivity. However, their high thermal conductivity limits their overall thermoelectric performance. To address this, Mg has recently been introduced as an alloying element at Cd atomic sites to reduce the lattice thermal conductivity of YbCd₂Sb₂. Zhang et al. have reported a high zT (a figure-of-merit for the thermoelectric performance) of 1.4 at 700 K in Yb(Cd_0.8Mg_0.2)₂Sb₂. They have demonstrated that the high zT is due to significantly suppressed phonon transport, in other words, low lattice thermal conductivity. They attributed the significantly low lattice thermal conductivity to severely distorted lattices that could not be described even with the Debye-Callaway model. Here, the Debye-Callaway model and Callaway-von Baeyer model have been utilized to evaluate the effect of Mg alloying on the lattice thermal conductivity of Yb(Cd_1-xMg_x)₂Sb₂ (x = 0, 0.1, 0.2) by estimating their theoretical lattice thermal conductivities. We found that appropriately fitting the parameter included in the phonon relaxation rate (of the Debye-Callaway model), which represents a fractional change of bulk modulus to that of local bond length, could describe the significantly suppressed lattice thermal conductivities of Yb(Cd_1-xMg_x)₂Sb₂ (x = 0, 0.1, 0.2). (Received 13 March, 2023; Accepted 20 April, 2023)

keyword : zintl phase, YbCd_{2Sb2, single parabolic band model, debye-callaway model, callaway-von baeyer model}

Lithium-ion batteries (LIBs) have undergone countless enhancements in the past decade, mainly improvements in the basic components: electrodes, electrolyte, and separator. The separator, which acts as a physical barrier between the two electrodes, does not directly participate in the charge and energy storage. However, it is involved in the safety, form factor, and packaging density of the LIBs. While it occupies relatively less internal space than other components, the separator can be replaced with active materials such as gel polymer electrolytes (GPEs) which can serve as both the electrolyte and physical barrier between the electrodes. GPEs can potentially minimize the risks of liquid electrolytes, including flammability, electrolyte leakage, and explosion. Here we report the characteristics of polyurethane (PU)-based gel swollen in concentrated electrolyte solutions in separator-less cells. The poreless PU-based gel electrolyte conducts lithium ions, while preventing internal short-circuits. This is attributed to the presence of soft segments, which allow ion transport, and hard segments, which ensure mechanical integrity. Electrochemical measurements carried out in LFP half cells and symmetric Li cells revealed that the separator-less cells were operable between 0.2 C to 1 C rates, and that during long term cycling, the cells achieved stable Li electroplating overpotential, as the number of cycles increased. (Received 29 March, 2023; Accepted 2 May, 2023)

keyword : lithium-ion batteries, LIBs, separator-less architecture, gel polymer electrolytes, GPEs, thermoplastic polyurethane, TPU, lithium bis, trifluoromethanesulfonyl, imide, LiTFSI